ATVs exist in different layouts and with different drive and steering systems. There are mainly two categories as to how steering can be effectuated on such vehicles.
In the first and most common category of such vehicles, the vehicles are steered by at least two wheels, which are pivoting horizontally around a vertical pivot point. These wheels are connected mechanically, by rods or gears to a steering wheel or a handlebar. The driver turns the handlebar or steering wheel to turn the wheels and to steer the vehicle. The steering can be assisted hydraulically or electrically to reduce the steering forces for the rider.
In the second category of such ATVs, the vehicles are skid steered and have no horizontal pivoting wheels. All wheels are solid mounted regarding the driving direction of the vehicle and are rigidly connected to a suspension system or directly to the vehicle body. This applies for wheel driven vehicles either with 4, 6, or 8 wheels as in FIG. 2, or for vehicles with endless tracks instead of wheels. The present invention pertains to this second category.
Such skid steered ATVs are known from different manufacturers in different vehicle classes, beginning with very heavy battle tanks and ending with light vehicles with a weight of about 1 ton at maximum, and including commercial snow handling vehicles and vehicles used in construction and the like.
The present invention pertains to a sector out of all possible skid steered vehicles, such as a full track ATV, as disclosed in the U.S. Pat. No. 7,131,507 B2 with the same inventors as this application, or light weight small vehicles with four, six or eight wheels, which are present on the market. The dryweight of those vehicles is under about one ton.
One of the drawbacks of the known ATVs of the prior art, and also of the low weight ATVs, hereinafter LATV, concerns the steering. In particular, smooth control and full security is required at higher vehicle speeds and, especially, at variable or changing ground conditions.
These light and relatively small LATV are usually steered in a way that requires the drive system to be divided into right wheels or track and left wheels or track. When steering is applied, the left or the right wheel group (or track) is disconnected from the drive with some kind of a clutch, and brake force is applied to that same wheel group or track to slow down the speed and to make the vehicle turn because of this resulting speed difference between the wheel groups or tracks of the vehicle.
The drawback of such systems is that the brake which is needed to steer the vehicle destroys the torque on one side of the vehicle, e.g., the inner side of the vehicle when driving a curve, whereas on the other side of the vehicle, i.e., the outer side, more torque and track speed is, needed to keep the vehicle speed constant. This system is inefficient and does not allow operating the vehicle smoothly and safely at higher speeds. It is especially important, if the driving ground condition changes from hard to soft (for example, from tar to snow or mud), when the vehicle is driven around a corner. During such change in driving conditions, these vehicles get out of control because the driver must adapt applied braking forces to compensate for the higher or lower grip and resistance behavior of the tracks or wheels, that is practically not controllable because of a very short reaction time involved.
To obviate the above mentioned drawback, it is known to use a differential steering device. There exist very heavy and very complicated differential steering systems for battle tanks and similar heavy vehicles. However, these cannot be used in light and small vehicles because of their complexity, weight, size and costs, as per the following examples.
U.S. Pat. No. 6,135,220 discloses a track-laying vehicle and is directed to a modular system for supporting the drive mechanism. This system comprises two drive units including a differential and a steering assembly using an additional differential.
U.S. Pat. No. 4,434,680 discloses a steering differential for earthmoving tractors, but this steering differential is not applicable for fast moving LATVs and is too complex for light and low cost vehicles.
WO 97/02975 discloses a differential driving system with two differentials and a steering differential.